The present invention relates to a bandwidth compression apparatus making possible bandwidth compression of speech signals in the state of analog signals, and in particular to a speech signal bandwidth compression and expansion apparatus suitable for analog transmission on narrow band radio transmission channels.
In recent years, use of radio transmission lines have gone on increasing. On the other hand, the radio frequency bands are finite resources. Therefore, compression of the occupied bandwidth is demanded strongly from not only the aspect of cost reduction but also the aspect of effective use of resources.
To take the instance of speech signal transmission as an example, the frequency band of human speech signals typically extends over several kilohertz although there is an individual difference. For transmission thereof, therefore, a transmission system having a frequency band of several kilohertz in the same way is needed. If the occupied bandwidth can be compressed without impairing articulation required for information transmission using speech, the cost required for the transmission system can be reduced.
From the past, therefore, various bandwidth compression techniques for speech signals have been proposed. In an example of known bandwidth compression techniques for speech signals, bandwidth compression of speech signals is attained by grasping the human vocal organ as a kind of autoregression system, simulating a speech signal as a signal generated by this autoregression system, and extracting system parameters by using prediction analysis. Examples are disclosed in the following papers.
(1) "Residual-excited linear prediction vocoder with spectral flattener utilizing the learning identification method (LI-RELP)", The Transactions of the Institute of Electronics, Information and Communication Engineers, vol. J68-A, No. 5, pp. 489-495, May 1985.
(2) "The residual-excited linear prediction vocoder with transmission rate below 9.6 kbit/s", IEEE Transactions on Communications, vol. COM-823, no. 12, December 1975, pp. 1466-1474.